The present invention relates to reducing surface roughness of chemical vapor deposition (CVD) films, and more specifically to the use of a multiple-step process to deposit a smooth film of metal on a semiconductor wafer at film thicknesses above about 200 xc3x85 in the fabrication of a semiconductor device.
Integrated circuits (IC) are often fabricated with one or more semiconductor devices, which may include diodes, capacitors, and different varieties of transistors. These devices are generally fabricated by creating thin films of various materials, i.e. metals, semiconductors or insulators, upon a substrate or semiconductor wafer. The terms wafer and substrate used in the following description include any semiconductor-based structure having an exposed surface with which to form an integrated circuit or semiconductor device, and may include one or more semiconductor layers or structures which includes active or operable portions of semiconductor devices. Wafer and substrate are used interchangeably to refer to semiconductor structures during processing, and may include other layers that have been fabricated thereon. The physical characteristics and tightly controlled placement of films on a substrate will define the performance of the semiconductor device and its surrounding circuitry.
Semiconductor fabrication continues to advance, requiring finer dimensional tolerances and control. Modern integrated circuit design has advanced to the point where line width may be 0.25 microns or less. As a result, repeatability and uniformity of processes and their results is becoming increasingly important.
One important process for depositing thin films on semiconductor wafers is chemical vapor deposition or CVD. CVD is used to form a thin film of a desired material from a reaction of vapor-phase chemicals containing the chemical constituents of the material.
CVD processes operate by confining one or more semiconductor wafers in a chamber. The chamber is filled with one or more reactant gases that surround the wafer. Energy is supplied within the chamber and particularly to the reactant gases near the wafer surface. The energy activates the reactant gas chemistry to deposit a film from the gas onto the heated substrate. Such chemical vapor deposition of a solid onto a surface involves a heterogeneous surface reaction of the gaseous species that adsorb onto the surface. The rate of film growth and the quality of the film depend on the process conditions.
CVD processing typically may be low-pressure CVD (LPCVD) or plasma-enhanced CVD (PECVD). The plasma used in the PECVD is a low-pressure reactant gas that is developed in a radio frequency (RF) field. The RF plasma results in a very high electron temperature, making possible the deposition of films at lower temperatures and faster deposition rates than are typically possible using purely thermally activated CVD processes.
Deposition of a film begins with nucleation as the atoms or molecules of the desired material begin to condense on the substrate and agglomerate to form nuclei. Growth of these nuclei will fill in the gaps between individual nuclei to develop a continuous surface or film.
To obtain the desired performance characteristics of a semiconductor device, the properties of the deposited films become critical. Because of their dimension, often less than 1000 xc3x85 in thickness, the properties of thin films are strongly dependent on their surface characteristics. This result stems from the substantial increase of surface-to-volume ratio of the film material as film thickness is decreased. Films with small grain structures may have more predictable performance characteristics than those with larger grain structures.
It is generally well known that films have smaller grain size at lower film thicknesses. As film thickness is increased, larger grains generally appear. These nominally thicker films, however, are often necessary to provide adequate step coverage where the deposition process encounters high aspect ratios or steps in the substrate topology. Due to the rapidly changing geography at these step interfaces, thinner films face a larger risk that the film will be too thin in some areas to achieve the desired performance characteristics.
CVD techniques for depositing platinum on substrates often follow this pattern of increasing grain size. At film thicknesses below about 200 xc3x85, CVD of platinum produces relatively smooth surface characteristics associated with small grain structure. As platinum film thickness is increased above about 200 xc3x85, larger grains begin to form producing less desirable surface characteristics.
In light of the foregoing, it may be desirable to form metal films with small grain structures at thicknesses above about 200 xc3x85 in the fabrication of semiconductor devices. The invention provides a CVD technique capable of forming such metal films with grain structures smaller than those of pre-existing techniques.
The invention allows the user to form metal films through CVD at reduced grain size over pre-existing techniques for one or more metals of the platinum group or noble metals. Smooth film growth is accomplished by exploiting the differing process conditions favorable to nucleation and agglomeration versus continued small grain growth. To achieve smooth films on substrates at film thicknesses above about 200 xc3x85, the invention discloses a multiple-phase CVD process whereby a first set of process conditions is utilized for nucleation, agglomeration and initial smooth metal film growth, and a second set of process conditions is utilized to continue smooth metal film growth.
In the first phase of the process of the invention, nucleation is initiated at a set of process conditions favorable to nucleation, agglomeration and initial smooth metal film growth. The set of process conditions in one embodiment may include pressure, oxidizer concentration and temperature. The process condition values favoring nucleation, agglomeration and initial smooth metal film growth are generally at higher levels than those favoring continued film growth with small grain structures. During this first phase, a first layer of metal film will be deposited upon the substrate or semiconductor wafer. The first process phase is allowed to continue until at least nucleation and agglomeration are complete, i.e., the first layer of metal film forms a continuous film. Subsequently, after nucleation and agglomeration are complete, but before large grain growth begins, one or more of the process conditions are altered to produce a set of process conditions favoring continued growth of a smooth metal film. Such continued growth of metal film can be thought of as depositing a second layer of metal film, although it is to be understood that the second layer of metal film deposited during the second process phase is contiguous with and uninterrupted from the first layer of metal film deposited during the first process phase.
Using the process and system of the invention, the user is capable of producing metal films on substrates or semiconductor wafers with total film thickness above about 200 xc3x85 having smoother surface characteristics, or smaller grain structure, over pre-existing CVD techniques. The user is further capable of producing semiconductor devices containing metal films with total film thickness above about 200 xc3x85 having smoother surface characteristics, or smaller grain structure, over devices produced using pre-existing CVD techniques.